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Which Schemes Should Be Used To Protect The Position Of The Control Valve Fault?
- Apr 23, 2018 -

The role of regulating valve in process control is well known, in the process of many control requirements regulating valve in failure at a certain position, in order to protect the process is not an accident, this requires the regulator in design to realize fault - safety of three broken (expired, cutting off power and signal) protection. For electric control valve, is simpler, fault signal, can be set according to the control module and stay at any position in the full open, close, maintain, and power outages, natural stay in the fault location, or with a reset device of electric actuator valve position will also be running to the fully open or close.

For the pneumatic control valve, the situation is more complicated, so we mainly discuss the three-break protection method of pneumatic control valve. In general, when choosing pneumatic diaphragm regulating valves, we all want to make sure of selected gas open or closed gas, this is to choose the regulator to snuff out the protection of the position, if technical requirements to snuff out the valve open, select valve normally open (gas) closed type, choose conversely normally closed valve (gas). This is just a superficial solution, if the technical requirements to snuff out three fault protection, cutting off power and signal, the regulator will need to configure some accessories to form a protection system to achieve control requirements, these attachments are mainly a valve, solenoid valve, tank, etc. The following are two kinds of protection schemes for single acting pneumatic film regulator and double acting pneumatic regulator.

1. Pneumatic film regulating valve scheme (power valve positioner for regulating valve)

The scheme is composed of pneumatic control valve, electric-pneumatic valve positioner, power loss (signal) comparator, single electric control electromagnetic directional valve, pneumatic position valve and valve position signal regenerator. Its working principle is as follows:

1, gas source: when the air supply control system failure, loss of air pressure, air will automatically shut down the position control valve positioner output signal of the pressure locking membrane of the pneumatic control valve interior, output signal pressure and valve spring force produced by the phase equilibrium, pneumatic control valve valve position in fault location. The position valve should be set at a minimum of the gas source.

2, power off: when the control system of a power failure (electricity) loss, loss of electricity (signal) comparator control the output voltage of the single electric solenoid directional valve to disappear, single electric solenoid directional control valve, the losing electricity within the single electric solenoid directional valve valve in the role of the reset spring sliding, electromagnetic valve, reversing the pneumatic membrane chamber pressure drain valve, pneumatic valve closed, the locator's output signal pressure locking membrane of the pneumatic control valve interior, output signal pressure and valve spring force produced by the phase equilibrium, pneumatic control valve valve position in fault location.

3, fault signal: when the control system signal failure (lost), the loss of electricity (signal) detected by the comparator, after breaking a single electric voltage signal of electromagnetic directional valve, electromagnetic directional valve lose electric single control system, single electric solenoid directional control valve in the valve in the role of the reset spring sliding, electromagnetic valve, reversing the pneumatic membrane chamber pressure drain valve, pneumatic valve closed, the locator's output signal pressure locking membrane of the pneumatic control valve interior, output signal pressure and valve spring force produced by the phase equilibrium, pneumatic control valve valve position in fault location.

The position feedback signal is given by the valve position signal.

The advantages of this scheme: when the "three-fault" protection starts, the system responds quickly and moves quickly. The overall cost is cheaper.

The disadvantages of this scheme: the long - term charge of the solenoid valve affects the service life. The accessories are more complicated, the installation and debugging are more complicated, and the valve bit feedback needs to be equipped with another valve position signal regenerator. In the case of hand wheel, it is more complicated.

2. Double-acting pneumatic regulating valve scheme (regulating valve with power-gas valve positioner)

The program consists of control valve, air control directional valve, positioner, self-locking valve, one-way valve, pressure reducing valve and gas storage tank. Its working principle is as follows:

When the air supply control system failure (loss of air pressure), self-locking valve (its action mode and contrary to the position control valve) opens automatically, control of pneumatic directional control valve of the gas source, pneumatic directional control valve of the valve under the action of the spring reset, one of the two pneumatic directional valve exhaust, another intake, one-way valve closed, air supply by air to the air supply valve, stored in the storage tanks to close or open the valve. The whole or open conversion can be realized by adjusting the connection mode of the air-controlled reversing valve.

If the valve protection position is to be realized, the valve shall be equipped with pneumatic position valve and the pipe connection shall be changed. The valve shall be directly controlled by the self-locking valve, and the air control valve, one-way valve and storage tank can be removed.

The following scheme can be used to ensure that the valve has a number of actions when the air source is broken.

It is composed of gas storage tank, one-way valve, blocking valve and globe valve. Its working principle is as follows:

When gas source faults (loss of air pressure), one-way valve closed, latching valve loss of air pressure, in latching valve valve under the action of the spring reset, pneumatic directional control, air piping that open system, through to storage tanks line, from storage tanks to the valve gas supply, to ensure that the valve has a number of times, the realization of the aim of continuous control. Because the capacity of the storage tank is limited, and the air source pressure in the storage tank decreases with the valve operation, it is not possible to use the storage tank to supply the valve in the long term. The capacity of the gas storage tank should be larger than that of general protection. The number of valve actuation is related to the capacity of the gas tank.

For confirmed a scheme of pneumatic diaphragm regulating valves, as well as a reference: between the positioner and actuator and the position control valve series (one, two of the 3-way magnet valve (when expired with to the position, the position control valve in the fault signal, the solenoid valve to the position, however, solenoid valve must be conducted with the locator chain (set) in the control program, namely the locator have signal, the solenoid valve will be electric, locator once lost signal, the electromagnetic valve must immediately without electricity.

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